Anisotropic optical spectra of PrBa2Cu4O8: Possible Tomonaga-Luttinger liquid response of the quasi-one-dimensional metallic CuO double chains

The optical spectra of PrBa2Cu4O8 show large in-plane anisotropy. For the a polarization (E perpendicular to chain), the spectrum is characterized by a gap of 1.4 eV indicating the charge-transfer insulating nature of the CuO 2 planes. For the metallic chain direction (E parallel to b), the spectrum deviates from a simple Drude response; reflectivity R-b(omega) shows a shar p edge at similar to1 eV but it also shows a dip at similar to 15 meV, whic h splits the conductivity spectrum into two parts-a zero-energy mode with s mall weight and a pronounced 40 meV made. These features are discussed in t erms of a Tomonaga-Luttinger liquid in a doped 1D Mott insulator and compar ed with 1D Bechgaard salts.